PH 205

What is magnetism? How does Earth create a magnetosphere? What effect does the magnetosphere have on the planet?

The magnet on the left is a horseshoe magnet. As you can see, it can pick up paper clips. It has a magnetic field because it has domains in the metal where the electron spins are aligned.

The magnet on the right is an electromagnet. It has a magnetic field that arises because of the current in the wire wrapped around the nail. Moving electric charges create a magnetic field.

The nail wrapped with insulated copper wire only acts as a magnet when the electric current flows through the wire. The motion of electrons in the wire creates the magnetic field.

Here we see iron filings suspended in oil. Again, the electromagnet only exerts a magnetic field on the filings when the current is on.

Earth has a liquid metallic outer core that is rapidly spinning. The motion of the charged particles in the outer core create a magnetic field that envelops the Earth, called a magnetosphere. The magnetosphere is not spherical like a ball. It would be shaped something more like this if it were not for outside influences.

Material streaming off the Sun creates a solar wind, consisting mostly of photons, protons and electrons. The solar wind pushes against Earth's magnetosphere, changing its shape.

Just as the motion of charged particles creates a magnetic field, a magnetic field affects the trajectories of charged particles. Earth's magnetosphere deflects charged particles such as protons and electrons. Protons from the Sun and elsewhere in outer space can be traveling extremely fast, and can be very hazardous to living organisms. Cosmic rays made of these rapidly moving particles can be detrimental to living tissue. Our magnetosphere largely protects us from these affects.

Occasionally, the Sun ejects a large blast of plasma from its surface, called a coronal mass ejection (CME). Usually this plasma is directed away from Earth, but sometimes we do happen to lie in its path.

This animation from NASA's Goddard Space Flight Center illustrates what happens when a coronal mass ejection collides with Earth. Earth's magnetosphere largely deflects the CME but becomes very elongated and can reconnect along the magnetic field lines. The reconnection can cause large displays of the Aurora Borealis and Aurora Australis. Coronal mass ejections can also be hazardous for satellites and their sensitive electronics.

The northern lights, or Aurora Borealis, is a phenomenon that occurs when charged particles from the solar wind collide with atoms in the atmosphere. The collisions cause electrons in the atmospheric gas to become excited, and then emit photons as they drop back down to ground state.

Various colors are attributed to the different kinds of gas present at different altitudes. The most common colors are green and yellow, caused by collisions with oxygen atoms in the atmosphere. Collisions with nitrogen atoms can result in red, violet and occasionally, blue auroras.

The effect is most often seen in upper latitudes, where the magnetic field lines channel the charged solar particles toward the magnetic pole. A similar phenomenon is seen around the south pole, called Aurora Australis.

The aurora can also be seen from space. Here is some video from the International Space Station showing the Aurora Borealis from above. This view makes it easier to see the stratification of colors with the height of the atmosphere.

In the simplest model, we approximate Earth's magnetic field as a simple field, similar to a bar magnet, with a North pole and South pole. But in reality, Earth's magnetic field is much more complex. The image above shows the strength of the magnetic field, using data taken by NASA satellites. What's more, the field is constantly changing. The North magnetic pole shifts over a hundred miles per year, and the speed at which it shifts appears to be increasing.

Earth's magnetic field is not static; it changes over time. Most notably, Earth's magnetic field undergoes oscillations where the north magnetic pole and south magnetic pole switch positions. A similar phenomenon is seen in the Sun, undergoing a complete cycle about every 22 years.

This timeline shows the polarity of Earth's magnetic field, in units of millions of years, with our present polarity shown in black. The magnetic field can be mapped by measuring the magnetism of iron-rich lava. Several sources were used in correlating this data, including volcanoes in Hawaii as well as measurements taken at the mid-oceanic trench. There, the drifting apart of the continents provides a steady source of new lava. The lava cools upon contact with the cold ocean water, freezing in the orientation of the magnetic field in the rock at the time of the solidification. As you can see, the polarity switching has not been regular in time. Sometimes it switches quite a few times in ten million years, other times the switches are much longer apart. Data taken in recent years leads physicists to believe that the magnetic poles of the Earth are approaching a switch. In fact, it may already be underway.

Using supercomputers, physicists have modeled the behavior of the magnetic field arising from the dynamo in its outer core. We believe that during magnetic reversals, the field becomes much more complicated. Although these models cannot be taken as an accurate prediction of what will happen, it does give us an idea of what to expect. The reversals appear to take hundreds or thousands of years to complete, depending to the model.

Experimentalists have approached the question in another way, by rapidly spinning a sphere containing twelve tons of molten metal to mimic the dynamo in a plant core. Experiments show that the magnetic field created by the spinning sphere actually does periodically reverse polarity, even though the sphere is kept spinning in the same direction.

How will the magnetic field reversal affect life on Earth? It may post a problem for birds like the bar-tailed godwit, which migrates from Alaska to New Zealand, some 6,000 miles nonstop across the ocean. The birds utilize air flows and, with no landmarks in the open water, probably find their way by sensing the magnetic field, as many other birds do.

As evidenced by the timeline of the field reversals, there is no correlation with extinctions. Physicists do not expect violent winds or dramatic effects. There could be anomalies like auroras appearing in southern latitudes, and at times in specific places, solar winds could make problems for satellites and power grids.